1. Field of the Invention
The present invention relates to a method for producing a nitride crystal, especially to a method for producing a nitride crystal through growth of a nitride crystal according to an ammonothermal process using a mineralizing agent that contains multiple types of halogen elements.
2. Description of the Related Art
An ammonothermal method is a method for producing a desired material using a nitrogen-containing solvent such as ammonia or the like in a supercritical state and/or a subcritical state and utilizing the dissolution-precipitation reaction of the starting material therein. In the method, when applied to crystal growth, a supersaturation state is generated through the temperature difference based on the temperature dependence of the solubility of the starting material in the solvent such as ammonia or the like, thereby precipitating a crystal. In a hydrothermal method similar to the ammonothermal method, water in a supercritical and/or subcritical state is used as the solvent for crystal growth, and the method is applied mainly to a crystal of oxide such as quartz (SiO2), zinc oxide (ZnO) or the like. On the other hand, the ammonothermal method is applicable to a nitride crystal, and is utilized for growth of a crystal of nitride such as gallium nitride or the like.
Gallium nitride crystal growth according to the ammonothermal method is a reaction in a supercritical solvent environment at high temperature and high pressure (500° C. or higher, 150 MPa or more), and in addition, the solubility of gallium nitride in a solvent such as pure ammonia or the like in a supercritical state is extremely low, and therefore, it is known to add an ammonium halide or the like as a mineralizing agent for the purpose of increasing the solubility and promoting the crystal growth. For example, Patent Reference 1 and Patent Reference 2 describe use of ammonium fluoride, ammonium chloride, ammonium bromide or ammonium iodide as a mineralizing agent.
On the other hand, it is known that, when gallium nitride is grown using an iodine-containing mineralizing agent or a bromine-containing mineralizing agent, then hexagonal-system gallium nitride crystals that differ in the crystal structure may tend to be formed, and it is suggested that these mineralizing agents are unsuitable to growth of useful hexagonal-system gallium nitride (Non-Patent Reference 1).